The cortical collecting duct (CCD) plays an important role in Na+ reabsorption and K+ excretion. Both K+ secretion and Na+ reabsorption involve several transport proteins including apical Na+ and K+ channels, Na-K-ATPase, and basolateral K+ channels. The basolateral K+ channels participate in generating cell membrane potential and play a critical role in K+ recycling which is important for maintaining the activity of the Na-K-ATPase. Preliminary experiments show that the basolateral small conductance K+ channel (SK) is involved in K+ recycling since the activity of the SK channels is closely related to the apical Na+ transport. That apical Na+ transport acts in concert with Na-K-ATPase and basolateral K+ conductance is important for maintaining a constant intracellular ion concentration and cell volume in the CCD. Therefore, the basolateral K+ channels must be regulated so that K+ recycling can match the apical Na+ transport. We have previously shown that the SK channels are activated by nitric oxide (NO) via a cGMP-dependent pathway. Furthermore, preliminary data demonstrated that inhibiting the apical Na+ transport reduces the activity of the basolateral SK channel activity and the effect of inhibiting Na+ transport depends on NO-cGMP signal transduction pathways. We propose to test the hypothesis that NO is critically involved in coupling the activity of the SK channels to the apical Na+ transport and the activity of Na-K-ATPase. The proposed studies have four Aims. Aim 1 will involve investigating the effects of NO on the SK channels and exploring the mechanisms by which NO regulates the SK channels. Aim 2 will study the role of NO in mediating the Ca2+ effects on the SK channels. Aim 3 will assess the role of NO in mediating the coupling mechanism between apical Na+ transport and basolateral K+ channels and between Na-K-ATPase and basolateral K+ channels. Aim 4 will examine the effect of Angiotensin II on the SK channel and investigate the role of NO in mediating the effect of angiotensin IL. Since the studies are conducted on freshly isolated CCD, the results will provide information essential for understanding the role of NO in the regulation of Na+ and K+ transport in the CCD.